The present invention relates to a projection optical unit for projecting an image on an image display element, enlargedly, upon a projection surface, such as, a screen or the like, and it relates, in particular, to a projection optical unit, being suitable for an image displaying apparatus of a front projection type, and further it relates to a projection-type image displaying apparatus and a projection-type image displaying system, applying such the projection optical unit therein.
In a color image displaying apparatus for projecting an image on an image display element, enlargedly, upon a screen (i.e., a projection surface) through a projection optical unit being constructed with a plural number of lenses, it is required to obtain an enlarged image having a sufficient largeness on the screen, but without distortion. Also, at the same time, for obtaining an easiness of handling of the apparatus, it is required to shorten a projection distance. For achieving this is already known an optical system for projecting an image, enlargedly, upon the screen, from an oblique direction (hereinafter, this is called an “oblique projection”), and as is described in the following Patent Documents 1 and 2, there is already known a projecting apparatus or an optical system for enlarging and projecting an image onto the screen, in the oblique direction, while shifting the projection screen into the vertical direction with respect to an optical axis of a projection system, with using an additional optical system, being also disposed inclining by a predetermined angle with respect to the optical axis of the projection. However, the additional optical system (e.g., an afocal converter) mentioned herein is an optical system having a function of converting sizes of a projection image, i.e., for obtaining an oblong projection image while compensating/reducing distortions of the projection image accompanying the projection into the oblique direction with respect to the screen.
Also, there is already known a reflection-type image forming optical system, for projecting an image on the image display element onto a screen (i.e., a projection screen), enlargedly, but with using a plural number of reflection mirrors (e.g., reflection-type optical elements) in the place of the lenses (e.g., transmission-type optical elements) mentioned above, for example, in the following Patent Document 3.
[Patent Document 1] Japanese Patent Laying-Open No. Hei 5-134213 (1993);
[Patent Document 2] Japanese Patent Laying-Open No. 2000-162544 (2000); and
[Patent Document 3] Japanese Patent Laying-Open No. 2004-157560 (2004).